Dilip dialyses a purified enzyme, which removes small molecules but keeps the protein…

Step 1. What it needed. It needed a cofactor, a small non-protein molecule. Without it, the apoenzyme has no activity. The small molecule from the extract supplied it.

Step 2. Which kind. It was removed by dialysis, so it was bound loosely. A loosely bound organic molecule is a coenzyme. (A metal ion can also be removed this way.) Heating the extract did not destroy it, as a small molecule is not denatured.

Step 3. The second enzyme. Dialysis did not remove anything it needed. Any cofactor it has must be bound tightly. That is a prosthetic group, such as haem.